The legacy YCbCr color format is a color-opponent, non-constant luminance format, where signals are interpreted based on color differences in an opposing manner. In YCbCr and similar color-opponent formats (such as YUV) the goal is to separate luma from chroma information for the purposes of chroma subsampling (i.e., 4:2:2 and 4:2:0). Chroma sub-sampling reduces the amount of data required to represent an image without affecting perceptually overall picture quality. Separating color from luma has also been proven to yield coding advantages in a variety of image and video coding standards, such as JPEG, MPEG-1, MPEG-2, AVC, HEVC, and the like.
Recently, High dynamic range (HDR) and wide color gamut (WCG) content have revealed the limitations of existing color encoding methods. Errors that were previously small with standard dynamic range can become magnified. Report ITU-R BT.2390-0 (2016), “High dynamic range television for production and international programme exchange,” which is incorporated herein by reference in its entirety, provides an alternative method for color difference encoding using a new color space referred to as the ICtCp (or ICTCP) color format.
Like YCbCr, ICtCp is a color-opponent based encoding scheme intended to separate luma from chroma information. In addition, ICtCp offers constant intensity (CI) representation. The CI neutral (grey) axis is encoded with the SMPTE ST 2084 or Hybrid Log-Gamma (HLG) non-linearity functions to match the human visual system, and to optimize it for high dynamic range signal encoding. Starting from RGB or XYZ representations, color transformation matrices to the ICtCp color format have been optimized for the human visual system perception of HDR and WCG content.
Most of the existing video compression standards, such as MPEG-1, MPEG-2, AVC, HEVC, and the like, have been tested, evaluated, and optimized for gamma-coded images in the YCbCr color space; however, experimental results have shown that the ICtCp color format may provide a better representation for high-dynamic range images with 10 or more bits per pixel per color component.
As used herein, the term “reshaping” refers to a pre-processing operation on an HDR image, such as scaling, quantization, and the like, to map it from its original bit depth to an image of a lower or the same bit depth to allow for more efficient coding using existing coding standards and devices. ‘Forward reshaping’ parameters used by an encoder may be communicated to a receiver as part of the coded bitstream using metadata so that a compliant decoder may apply an ‘inverse’ or ‘backward reshaping’ operation to reconstruct the original signal at its full dynamic range. Reshaping may be applied to any or all of the color components of an HDR signal. To improve existing and future coding standards, as appreciated by the inventors here, improved techniques for the chroma reshaping of HDR video are needed.
As used herein, the term “metadata” relates to any auxiliary information that is transmitted as part of the coded bitstream and assists a decoder to render a decoded image. Such metadata may include, but are not limited to, color space or gamut information, reference display parameters, and auxiliary signal parameters, as those described herein.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.